1. Field of the Invention
The present invention relates to a light source unit used for reading optical disks and the like. More specifically the invention is directed to a laser light source unit which provides a harmonic of laser light generated by a laser source which provides the fundamental wavelength of laser light.
2. Description of the Prior Art
Typical devices used to convert the wavelength of light from a fundamental wave to a secondary harmonic wave include the optical-fiber, optical-waveguide, and the bulk type light-wavelength conversion modules. In each, a fundamental wave produced by a low-output semiconductor laser light source or the like, utilizing inter-band transition of a semiconductor, is directed into a wavelength conversion element. The conversion element then outputs the second harmonic wave of the fundamental wave, which is then used, for example, to read an optical disc.
The nonlinear optical effect is the phenomenon by which the second harmonic wave, the sum-frequency wave, the differential-frequency wave or the like are created. The nonlinear optical effect is the phenomenon that polarization proportional to the quadratic and higher-power terms of the electric field of light takes place when the light enters into a medium.
Materials in which such a phenomenon are generated are called nonlinear optical materials. Inorganic nonlinear optical materials such as KH.sub.2 PO.sub.4, LiNbO.sub.3, LiTaO.sub.3, and the like are well known. Furthermore, organic materials represented by 2-methyl-4-nitrileaniline (MNA), 4-dimethylamino-3-acetoamidnitrobenzene (DAN), and 3, 5-dimethyl-1-(4-nitrophenyl) pyrazole (DMNP) have recently been found to be suitable nonlinear optical materials because of their large nonlinear optical constants.
In order to create secondary harmonic waves from the fundamental wave, it is important that the converter is designed to confine the fundamental waves of laser light at a high density of energy and have a large length of interaction with the fundamental waves.
Such a converter includes the light wave passage type where a slender light wave passage portion through which light is propagated while being confined therein is formed on a base plate that is coated with an upper layer. See FIG. 6. To pass secondary higher harmonics generated in the light wave passage portion or the like, the light wave passage of the converter needs to have such constitution as to cope with the propagation phase velocity of the secondary higher harmonics generated from the fundamental waves, or the fundamental waves and the secondary higher harmonics need to be matched with each other in phase. There are various conceivable methods for such matching. The simplest of the methods is the Cerenkov radiation method.
Referring to FIG. 6 for a converter of the light wave passage type, suppose that secondary higher harmonics are generated, at a point A, from light being propagated in a light wave passage portion 91, and go out of the portion 91 into a base plate 92 and an upper layer 93 at an angle .alpha. thereto, and other secondary higher harmonics are generated, at another point B, from the light a unit time after the generation of the former harmonics, and proceed at the same angle .alpha.. If the equal phases of the secondary higher harmonics proceeding from the point A coincide with the equal phases of the other secondary higher harmonics proceeding from point B, both sets of harmonics go out of the light wave passage portion 91 at the angle .alpha. thereto. This phenomenon is called Cerenkov radiation. If n.sub.S (w) the refractive index of the base plate 92 to the fundamental waves of the light, n.sub.G (w) the refractive index of the wave passage portion 91 to the waves, and the n.sub.S (2w) the refractive index of the base plate to the secondary higher harmonics have a relationship expressed by the inequality EQU n.sub.S (2w)&gt;n.sub.G (w)&gt;n.sub.S (w) (1)
then the fundamental waves and the higher harmonics are automatically matched with each other in phase to enable the Cerenkov radiation. However, since secondary higher harmonics which are radiated from the light wave passage portion 91 into the base plate 92 of larger thickness are diminished, the light proceeding out of the end portion has a crescent cross-section. It is difficult to condense the light of the crescent cross-section to a small spot. For that reason, it is difficult to use a wavelength converter of the light wave passage type for a light source unit which is to be used for purposes such as writing and reading information to and from an optical storage medium including an optical disk.
Referring to FIG. 7 for a wavelength converter 100 which is of the optical fiber type and in which the refractive indices of the core 101 and outer portion 102 of the converter have the same relationship as those of the FIG. 6 light wave passage portion 91, base plate 92, and upper layer 93 as expressed by inequality (1), secondary higher harmonics 103 going out from the end surface of the converter spread to form a rotatively symmetrical ring on the front of the converter, as shown in FIG. 7. For that reason the light condensing property of the optical fiber type converter is good. The harmonics 103 can be easily collimated into mutually parallel rays by a conical lens or a Fresnel lens. The rays can be condensed to a diffraction-limit light spot. Since the wavelength converter of the optical fiber type is not only good in light condensing property but also small in size and weight, it can be expected to be integrated with a semiconductor laser module in a compact package so that the assembly is used as an optical disk reading light source unit or the like.
A light source unit including a wavelength converter of the optical fiber type was disclosed in the Japan Patent Application (OPI) No. 73327/89 (the term "OPI" as used herein means an "unexamined published application"). FIG. 8 shows the constitution of the light source unit in which laser light generated by a semiconductor laser module 21 is collimated into mutually parallel rays by a collimation lens 22. Since the beam of the rays has an elliptic cross-section, the section is shaped into a circular one by an anamorphic pair of lenses 23. After the shaping, the direction of the polarization of the laser beam is altered by a phase difference plate 25. The beam is then condensed, and entered into the core 11 of the wavelength converter 10 through the light incoming and surface 10a thereof. The converter 10 includes the core 11, and an outer portion 12 surrounding the core. The refractive indices of the core 11 and the outer portion 12 have the same relationship as those of the light wave passage portion 91, the base plate 92, and the upper layer 93 as expressed by the inequality (1). In the converter 10, the laser beam acts as fundamental waves for Cerenkov radiation so that secondary higher harmonics are generated and go out as converted light from the light outgoing end surface 10b of the converter before spreading to form a ring on the front of the harmonics. The converted light is condensed by a condensation lens 27, and then removed of fundamental wave components by a band-pass filter 28. The converted light is finally used for the reading of an optical disk or for the like.
Since the anamorphic pair of lenses 23 are used to shape the elliptic cross-section of the beam coming out from the collimation lens 22, the optical path of the beam is inevitably bent. For that reason, the light source unit cannot be made compact as a whole, causing a great disadvantage if the unit is to be used for an optical disk reading pickup or the like.